This is found on the Billing/Shipping Information page of the cart. It is used as the background to the table header that divides the customers billing information.

It also controls the background color of the product information area on the View Cart page.

You may change this color by selecting a specific color from the drop down menu. Or, if you would like to use a color that is not listed in this menu then choose "other/hex value" from the drop down menu. You will then need to supply the hexadecimal code (HTML color code) for the desired color in the box marked "other".